Flat Roofs in General
In the roofing art, and in this specification, "flat roof" refers to a roof having a slope of less than about 25.degree. relative to a horizontal plane. Many such roofs are substantially flat with a slight incline to allow water to run off. Some flat roofs comprise numerous sloping sections which create peaks and valleys, and a water drain is generally located at the bottom of each valley to facilitate water drainage. Flat roofs traditionally comprise three basic components (from top to bottom): (1) a waterproof membrane (top); (2) thermal insulation (middle); and (3) the structural deck (bottom).
The waterproof membrane typically comprises two or more plies of a felt membrane in combination with bitumen (generally coal tar pitch or asphalt). The felt stabilizes and strengthens the bitumen, and distributes contractive tensile stress when the bitumen is cold and glasslike. Alternatively, the membrane can be a polymeric sheet or a series of polymeric sheets adhered together to form seams where they are joined.
The membrane is typically used in combination with metallic and/or nonmetallic flashings which guard against leakage through portions of the membrane which are pierced or terminated, such as at gravel stops, walls, curbs, expansion joints, vents and drains.
Mineral aggregate (normally gravel, crushed rock, or slag) is often spread atop the membrane to hold it down on the roof deck and protect the membrane from wind, rain, solar radiation, and fire. Such aggregate may be unnecessary on smooth-surfaced asphalt roofs having glass-fiber felts.
Conventional membranes cannot resist large movements of the deck, or insulation covering it, and will be punctured by heads of fasteners which protrude above the insulation due to such movements. Membrane puncture (due to fastener heads, foot traffic or the like) and undue membrane shifting or movement (due to foot traffic, wind forces or the like) are primary causes for leaks in flat roofs which have been properly installed.